As used herein, the term "fluorescent" is intended to include other gaseous discharge lamps such as high-intensity discharge lamps. It is known that these lamps operate more efficiently at frequencies higher than 60 Hz. Typically, such frequencies may range from 15 KHz to as high as 100 KHz. In the prior art, there have been many suggestions of electronic ballast circuits capable of high frequency operation, and there have also been commercial attempts to provide such electronic ballasts.
It has recently been recognized that to obtain longer life on fluorescent lamps designed for high frequency operation, the lamp current should be regulated to a degree higher than some may have thought necessary in earlier ballast circuits. There is a problem in obtaining a highly regulated lamp current because, in a ballast intended for residential or commercial use, conventional 60 Hz line voltage is the only practical source of power. Even when full-wave rectified, so that a 60 Hz source in effect becomes a 120 Hz source, there is substantial variation in the amplitude of the source voltage fed to the power transformer which normally energizes the lamp load. If this variation in amplitude is reflected in applied lamp current, it is undesirable because it is believed any such variation will reduce effective lamp life.
A measure or factor sometimes used by lamp manufacturers to limit or define operating specifications of a ballast to ensure longer lamp life is the "crest factor" which is defined as the ration of the peak amplitude of the lamp current to the rms value of lamp current. Some lamp manufacturers require, at least for certain lamps, that the crest factor of a solid state ballast operating at high frequency be less than 1.6. Such crest factors have not been attainable by the circuit as disclosed in my above-identified application Ser. No. 194,783. One of the reasons that desirable values of crest factor are not obtainable by the circuit as disclosed in that application is that the voltage is permitted to go to zero volts between the cusps of the full wave rectified source voltage. (This period between the peaks or cusps of the rectified voltage, when the voltage goes to zero volts, or to carry-over voltage if any auxiliary supply is used, is referred to as the "inter-cusp" period.) Even if an auxiliary source of DC voltage is used to store energy (as in a capacitor, for example), crest factors of less than 1.6 are nevertheless difficult to attain in such a system without auxiliary filters or unduly large values of components, which results in larger sizes of components, and that defeats some of the primary purposes of a solid state ballast--namely, reduction in the size of components and increased operating efficiency.
In some electronic ballasts, the crest factor is improved simply by adding large filters to the full wave rectified source voltage, but, in turn, adding bulky and costly components and reducing the overall efficiency of the ballast.
Thus, a principal object of the present invention is to provide an electronic ballast circuit for fluorescent lamps which regulates lamp current such that the crest factor of lamp current achieves a value of less than 1.6. It is further object of the present invention to do so without adding bulky and expensive filter components for filtering the source voltage.